This study will be concerned with an attempt to refine our understanding of the role of birth order and family size for intellectual competence and for health status. Our previous findings, for a total population of some 400,000 19 year old Dutch males, based on the military preinduction examination, showed clearly that birth order and family size had independent effects on a number of psychological functions (perceptual reasoning, language competence, numerical ability, perceptual speed and mechanical comprehension). Further analyses will explore the pattern of abilities shown by individuals at given birth order positions within specific family sizes, with social class controlled. Because of the consistent nature of our findings (declining test score with higher birth order position and larger family size) we propose to extend our analyses on this population to a consideration of additionl variables which are pertinent to an interpretation of birth order effects. These variables include education, health status and stature, and information on sibling sex. The characteristics of two sub-groups, those from one-child families and those with mild mental retardation, will be given special consideration. Supplementary data collected on two-child families for a sub-sample will permit us to examine the additional variables of spacing and sex of sibling and maternal age in relation to the same set of characteristics.